Invited Lecture 2.1

Abstract

Current advanced treatment technique for radiotherapy is the volumetric-modulated arc therapy (VMAT).  This treatment technique utilizes rotational modulated radiation beams to deliver radiotherapy.  In addition to beam modulation, the rotational speed and dose rate also vary as the radiation beam rotates around the patient.  Since the dose delivery involves temporal components, the dose pattern can be different from that designed using treatment planning system.   Recently, we underwent credentialing of our new conventional medical linear accelerator to participate in cooperative group clinical trials.  We participated in the irradiation of three anthropomorphic phantoms: (a) lung phantom, (b) stereotactic brain phantom, and (c) head and neck phantom using VMAT.  Measurements in these anthropomorphic phantoms were made using TLDs and chromic films.  Treatment planning, phantom set up, and phantom irradiation follow established clinical protocols in our institution, and hence are carried out by our multidiscipline team members.  The measurements using TLDs and films were returned to the Imaging and Radiation Oncology Core (IROC) for dose determination and analysis. These anthropomorphic phantoms assess the effect of (a) tissue inhomogeneity, (b) small targets, and (c) the intensity modulated concept where two or more target volumes are present near an organ at risk.  The dose determination and analysis by IROC, the entity that oversees quality assurance for the cooperative group clinical trial, is based on a set of passing criteria on dose and gamma index.  The dose and gamma index passing criteria have been published by IROC.  This exercise on the irradiation of anthropomorphic phantoms proved to be useful to resolve unknown issues and bestowed confidence in the dose delivery system and the quality treatment following the clinical protocols.  Quality audit based on the irradiation of anthropomorphic phantoms is more robust in the assessment of dose delivery system than the mail-in TLD on machine output practiced worldwide.